<UserControl x:Class="MasterDetail.Views.StartView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:local="clr-namespace:MasterDetail.Views"
             xmlns:vm="clr-namespace:MasterDetail.ViewModels"
             Background="#FFFFFFFF"
             d:DataContext="{d:DesignInstance Type=vm:StartViewModel, IsDesignTimeCreatable=True}"

 // Добавлено !!!!
        public StartViewModel()  
        {
            // _appController = appController ?? throw new ArgumentNullException(nameof(appController));

            //команды
            //EditCommand = new RelayCommand(OnEdit, CanEdit);
            //DeleteCommand = new RelayCommand(OnDelete, CanDelete);
            //CreateCommand = new RelayCommand(OnCreate, CanCreate);

            //загружаем из БД людей
            LoadPeople();
        }

public class StartViewModel : IStartViewModel
{
        public event PropertyChangedEventHandler PropertyChanged;


        private readonly IAppController _appController;

        //ctor
        public StartViewModel(IAppController appController)
        {
            _appController = appController ?? throw new ArgumentNullException(nameof(appController));

            //команды
            EditCommand = new RelayCommand(OnEdit, CanEdit);
            DeleteCommand = new RelayCommand(OnDelete, CanDelete);
            CreateCommand = new RelayCommand(OnCreate, CanCreate);

            //загружаем из БД людей
            LoadPeople();
        }

      // Добавлено !!!!
        public StartViewModel()  
        {
            // _appController = appController ?? throw new ArgumentNullException(nameof(appController));

            //команды
            //EditCommand = new RelayCommand(OnEdit, CanEdit);
            //DeleteCommand = new RelayCommand(OnDelete, CanDelete);
            //CreateCommand = new RelayCommand(OnCreate, CanCreate);

            //загружаем из БД людей
            LoadPeople();
        }